This application claims the priority right under 35 U.S.C. 119 of Japanese Patent Application No. 95687/2000 filed on Mar. 30, 2000, the entire disclosure of which is incorporated by reference.
1. Field of the Invention
The present invention relates to an infrared sensor and a manufacturing method of the infrared sensor, and more particularly, it relates to a pixel structure of a uncooled infrared sensor and a manufacturing method of the structure, and provides a high-sensitivity uncooled infrared sensor and a manufacturing method of the infrared sensor.
2. Related Background Art
Infrared image pickup has characteristics that the image pickup enables day and night, and that an infrared ray is higher in transmittance to smoke or fog than a visible ray. Furthermore, temperature information of a subject can also be obtained. Therefore, the infrared image pickup has a broad application range in a security field and as a monitor camera and a fire detection camera.
As a largest defect of a quantum type infrared solid image pickup apparatus which has been a conventional main type, a cooling mechanism is necessary for a low-temperature operation. In recent years, however, xe2x80x9cuncooled infrared solid image pickup elementxe2x80x9d requiring no cooling mechanism has well been developed. The infrared solid image pickup apparatus of the uncooled, i.e., a thermal type converts an incident infrared ray with a wavelength of about 10xcexc to heat by an absorption structure, and then converts a temperature change of a heat sensitive portion caused by the slight heat to an electric signal by some thermoelectric conversion means, and then reads out this electric signal to obtain infrared image information.
In order to enhance sensitivity of such uncooled infrared sensor, there are three largely classified methods.
In a first method, a ratio of a power change dP of an infrared ray incident upon an infrared detector for temperature change dTs of a subject, that is, dP/dTs is enhanced. In this method, the sensitivity is enhanced mainly by an optical system. This method includes enlargement of an aperture diameter of an infrared lens, use of an reflection (AR) coating, use of a low-absorption lens material, enhancement of infrared absorbency of the infrared detector, enlargement of an infrared absorption area, and the like.
In recent years, the uncooled infrared sensor has tended to have multiple pixels. Moreover, a unit pixel size is mainly about 40 xcexcmxc3x9740 xcexcm. In the aforementioned problems, the enlargement of the infrared absorption area in the infrared detector remains to be relatively important.
However, it has been reported that the infrared absorption area is enhanced to about 90% of a pixel area by laminating/forming infrared absorption layers on an upper portion of the pixel (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p.556, 1999). It is difficult to further enhance the sensitivity by optical means.
In a second method, a ratio of the incident infrared power change dP to an infrared detector temperature change dTd, i.e., dTd/dP is enhanced. While the first method is an optical technique, the second method is a thermal method. Generally, in the uncooled infrared sensor mounted on a vacuum package, for transport of heat to a support substrate from the infrared detector, thermal conduction of a support structure for supporting the infrared detector in a cavity structure inside the support substrate is dominant now. Therefore, a leg-like support structure formed of a material with a low thermal conductivity is designed to be thinner and longer in a layout within a possible range (e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p.556, 1999).
However, the pixel size is reduced to about 40 xcexcmxc3x9740 xcexcm, and a fine processing is performed at a silicon LSI process level. Therefore, it is difficult to further enhance the sensitivity by devising the layout of the support structure. Similarly, it is also difficult to further reduce the thermal conductivity as one of material characteristics of the support structure. Particularly, for a wiring for outputting the electric signal from the infrared detector, there are contrary requests for electric conduction and heat conduction which are similar to each other in mechanism, and it is also difficult to realize a remarkable sensitivity enhancement in respect of the material.
In a third method, a ratio of an electric signal change dS caused by the thermoelectric conversion means to temperature change dTd of the infrared, i.e., dS/dTd is enhanced, and this is an electric method. Different from the other two methods, in the third method, simple sensitivity enhancement, i.e., enhancement of dS/dTd is an object, but it is very important to reduce various electric noises which are simultaneously generated. Various thermoelectric conversion means have been studied.
Main means are as follows.
(1) Thermopile for converting a temperature difference to a potential difference by Seebeck effect
(e.g., Toshio Kanno, et al., Proc. SPIE Vol. 2269, pp. 450 to 459, 1994)
(2) Bolometer for converting a temperature change to a resistance change in accordance with a change temperature of a resistor
(e.g., A. Wood, Proc. IEDM, pp. 175 to 177, 1993)
(3) Pyroelectric element for converting the temperature change to a charge by a pyroelectric effect
(e.g., Charles Hanson, et al., Proc. SPIE Vol. 2020, pp. 330 to 339, 1993)
(4) Silicon pn junction for converting the temperature change to a voltage change in accordance with a constant forward-bias current
(e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p.556, 1999, hereinafter xe2x80x9cIshikawa et alxe2x80x9d)
However, when comparing respective systems with one another, and considering all of thermoelectric conversion characteristics, noise characteristics, and manufacturing methods, it cannot be said under existing circumstances that there is a system decisively superior to the other systems. For example, the bolometer is superior in respect of temperature resolution, but the silicon pn junction is superior in respect of manufacturing processes because this junction can be manufactured only with a conventional silicon LSI process.
As described above, as one method for enhancing the sensitivity of the uncooled infrared sensor, there is the thermal method, in which the ratio of the infrared detector temperature change dTd to the incident infrared power change dP, i.e., dTd/dP is enhanced.
Generally, for the heat transport to the support substrate from the infrared detector, the heat conduction of the support structure for supporting the infrared detector in the cavity structure inside the support substrate is dominant. The leg-like support structure of the material having the low thermal conductivity is designed to be thinner and longer in the layout within the possible range. However, the pixel size is reduced to about 40 xcexcmxc3x9740 xcexcm. In such minute size, the fine processing is already performed at the silicon LSI process level. Therefore, it is difficult to realize remarkable sensitivity enhancement by devising the layout of the support structure.
The present invention has been developed based on recognition of the problems, and an object thereof is to provide an infrared sensor and a manufacturing method of the infrared sensor in which a sectional area of a support structure for supporting an infrared detector is remarkably reduced as compared with a conventional sectional area, and detection sensitivity is considerably improved by inhibiting heat xe2x80x9cescapexe2x80x9d.
To achieve the aforementioned object, there is provided an infrared sensor comprising:
a semiconductor substrate having a plurality of concave portions;
a plurality of infrared detectors formed above said semiconductor substrate, each of said infrared detectors having an absorber for absorbing an incident infrared ray to convert the incident infrared ray to heat, and a thermoelectric converter for converting a temperature change caused by the heat generated in said absorber to an electric signal, said thermoelectric converter including a semiconductor layer formed under said absorber, said pn junction being formed in said semiconductor layer to convert the temperature change to the electric signal; and
one or more support members for supporting each of said plurality of infrared detectors in corresponding one of said concave portions and apart from said semiconductor substrate, said support members extending in a direction substantially parallel to a surface of said semiconductor layer.
one end of each of said support members is connected to corresponding one of said infrared detectors and the other end thereof is connected to said semiconductor substrate, and
a thickness of each of said support members is smaller than that of corresponding one of said infrared detectors.
According to the present invention, in the aforementioned constitution, the support member disposed between the infrared detector and the semiconductor substrate can be formed to be considerably thin. Even when a layout limited by a fine processing level is the same, a sectional area of the support member can remarkably be reduced. Therefore, heat conduction of the support member, which controls heat transport between the infrared detector and the support member, can remarkably be reduced. As a result, a high-sensitivity uncooled infrared sensor can be obtained.